1. Field of the Invention
The present disclosure relates to a method for controlling an inverter, and particularly, to a control method used in an inverter system driving a plurality of motors.
2. Background of the Invention
In an application field, such as a fan, a pump, or the like, in which a flow rate and an oil pressure are mainly controlled, a plurality of motors control a flow rate or an oil pressure. Here, in general, a single controller controls a plurality of motors.
A controller such as an inverter, or the like, receives a feedback of a control amount of a process and controls a variation, and if necessary, an auxiliary motor may be selectively operated all the time to uniformly maintain a feedback control amount.
FIG. 1 is a view illustrating a motor system controlled by a general inverter.
As illustrated in FIG. 1, the motor system includes an Alternating Current (abbreviated as AC hereinafter) power source 1, an inverter 2 controlling a motor 3, the motor 3, and an electric load (abbreviated as load hereinafter) 4.
The inverter 2 includes a converter 21 that converts AC into Direct Current (abbreviated as DC hereinafter), an initial charging resistor 22 that prevents an inflow of a surge current when electric power is applied, a switch 23 that separates the initial charging resistor 22 from a circuit after the surge current is restrained, a filter condenser 24 that smoothes a DC voltage, a pulse width modulation (abbreviated as PWM hereinafter) inverter unit 25 that includes a plurality of switching elements and converts a DC into an AC, a current detection unit 26 that detects an electric current for each of 3 phases, so called U, V, and W phases, a controller 27 that collects various types of information such as a DC voltage, a phase current, and the like, and instructing various operations, and a PWM controller 28 that generates a PWM signal by using a voltage command V* and a frequency command f* input from the controller 27 and applying a switching signal to each switching element for each phase of the PWM inverter unit 25.
The load 4, which is actually operated by the motor 3, feeds back a load amount to the controller 27.
In the foregoing configuration, upon receiving electric power from the AC power source 1, the inverter 2 converts an output voltage and an output frequency through power conversion, and supplies the same to the motor 3 to control a speed and a torque of the motor 3 efficiently.
The inverter 2 precisely controls a speed of the motor 3 to save energy and enhance energy quality, so it is commonly used in automation facilities such as various air blowers, pumps, machine tools, textile machinery, and the like.
In general, the inverter 2 controls a main motor by using a proportional integral and derivative (abbreviated as PID hereinafter) controller, upon receiving a control amount, as a feedback, of a process, and if necessary, the inverter 2 controls an external signal such that an auxiliary motor is selected to be operated all the time to uniformly maintain the feedback control amount.
In such a system, while a plurality of motors are being controlled, when a pre-set flow rate or oil pressure is insufficient or excessive so the load cannot be controlled by only a main motor, the inverter 2 actuates the auxiliary motor.
In this case, when the main motor is controlled, PID controlling is performed. When a load amount is small, a plurality of motors controlled by the inverter 2 enters a sleep mode to save energy.
In a system illustrated in FIG. 1, references for determining an application of a sleep mode and sequences are as follows.
(1) The plurality of motor controlled by the auxiliary motor should be in a halt state.
(2) The operating frequencies of a plurality of motors should be lower than is a sleep frequency satisfying a sleep condition.
(3) A feedback should be lower than a sleep level (Namely, the load amount should be small).
(4) More than a delay time has lapsed in a stable system (There is a band with respect to a change in the load amount).
In the related art system, when the foregoing conditions are met, energy can be saved through a sleep operation and a wake-up operation.
If the controller 27 of the inverter 2 determines that a control amount required for the load 4 is sufficient, the controller 27 stops outputting and the inverter 2 may stop the system. Here, a sensor (not shown) for detecting a load amount is periodically checked, and when the load amount is increased again the controller 27 starts to perform an operation appropriate for the load 4 by starting the inverter 2.
FIG. 2 is a timing view illustrating a sequence with respect to the sleep and wake operations of the inverter, in which the sleep operation and wake operation are repeated according to the foregoing method.
Namely, in the related art system, in general, the sleep operation and the wake-up operation are performed only through the operations of FIG. 2, in the load 4 which is generally insensitive to a change in a load amount in most cases.
However, in case of a load sensitive to a change in a load amount, or in case of a load which cannot manage a change in a load amount only by the band with respect to a change in the load amount, the sleep operation and the wake-up operation are repeated.
Also, according to circumstances, even when a sleep level and a wake-up level set to be relative to the sleep level are not appropriately set, a continuous abnormal operation of a mode occurs.